Conventionally two types of P. C. steel rods are known, as illustrated in FIGS. 2a, 2b, 3a and 3b, a so-called indented rod which is a steel rod 13 in which round concave depressions 14 are formed at specified intervals in the longitudinal direction thereof, and a so-called P. C. strand 15 which is made up of a plurality of more than two steel wires 16 twisted together.
As will be explained hereinafter, the indented rod has the drawbacks that differences in their lengths after they are drawn out, and the depressions are not very effective in increasing the adhesion to the concrete. As for the P. C. strand, as will also be explained hereinafter, it is hard to achieve the prescribed tension load, using this type of bar.